The lipid A induced secretion of non-immunoglobulin protein by bone marrow cells derived from responder, nonresponder and low responder mouse strains was observed to correlate poorly with the lipid A responsiveness assessment based on the mitogenic reactivity of sphenocytes. This new phenotype should prove useful in characterizing the lipid A response in regard to many biological activities that are known to be controlled at a single genetic locus in responder/nonresponder mice. A simple method has been devised using a single glass surface for purification, identification and autoradiographic analysis of 125I-lipid A binding by individual macrophage cells derived from spleen and peritoneal fluids. It has been demonstrated that ATP covalently linked to agarose is mitogenic for T cells at a nominal ATP concentration one tenth the concentration required to achieve the same effect with soluble ATP. The findings are compatible with a membrane site for ATP function as a mitogen.